In recent years, with the rapid spread of information-related devices and communication devices such as personal computers, video cameras and cellular phones, it has been regarded as important to develop a cell that is used as the power source of such devices. Also in the automobile industry, the development of a high-power and high-capacity cell for electric vehicles and hybrid vehicles has been promoted. Among various kinds of secondary cells, a lithium cell and a lithium-ion cell have been attracting attention because of their high energy density and output.
The lithium-ion cell, which is mainstream now, uses a combustible organic solvent as an electrolyte, however. Accordingly, safety measures against a possible liquid leak, short circuit, overcharge and so on are necessary. For improved safety, a solid lithium-ion cell that uses a solid electrolyte as an electrolyte, such as an ion-conducting polymer or ceramic, has been developed. Especially, sulfide-based inorganic solid electrolytes such as a sulfide glass and a crystallized sulfide glass have been attracting attention as the ceramics which are usable as the lithium-ion conducting solid electrolyte because of their high lithium-ion conductivity.
In general, solid-state cells typified by a solid lithium-ion cell comprise a positive electrode, a negative electrode, and a solid electrolyte layer disposed between the electrodes. The positive and negative electrodes are formed by using an electrode active material only or by using, in addition to an electrode active material, a solid electrolyte for imparting ion conductivity to the electrodes or an electroconductive material for imparting electroconductivity to the same. The solid electrolyte layer is formed by using a solid electrolyte only or by using, in addition to a solid electrolyte, a binder for imparting flexibility to the solid electrolyte layer, etc.
In the method for producing the electrodes of a solid-state cell, for example, as the method for forming the electrodes, there may be mentioned a method in which an electrode active material is mixed with a solid electrolyte, an electroconductive material, etc., as needed to prepare an electrode material powder, and the electrode material powder is press-formed by powder compacting. As the method for producing a solid electrolyte layer, there may be mentioned a method in which a solid electrolyte is mixed with a material such as a binder as needed to prepare an electrolyte material powder, and the electrolyte material powder is press-formed by powder compacting.
Methods other than the powder compacting method include, for example, a method in which the above-mentioned electrode material powder or electrolyte material powder is dispersed in a solvent to prepare a slurry, and the slurry is applied to a substrate (such as a removable substrate, an electrode or the like) and dried, thereby forming each electrode or a solid electrolyte layer.
Specific methods for producing a solid-state cell by powder compacting include, for example, methods disclosed in Patent Literatures 1 and 2.
Patent Literature 1 discloses a method for producing an all-solid-state lithium secondary cell by press-forming a solid electrolyte layer at a temperature that is equal to or less than the softening temperature of the solid electrolyte and equal to or more than the glass-transition temperature of the same. In Examples of Patent Literature 1, a solid electrolyte layer formed by press-forming a solid electrolyte powder is pressed under a heating condition in the state in which the layer is sandwiched between a positive electrode formed by press-forming a positive electrode powder prepared by mixing a solid electrolyte powder with a positive electrode active material, and a negative electrode formed by press-forming a negative electrode powder prepared by mixing a solid electrolyte powder with a negative electrode active material.
Patent Literature 2 discloses an all-solid-state lithium cell comprising a pair of electrode layers and a solid electrolyte layer, in which at least one of the pair of electrode layers and the solid electrolyte layer are integrated with an electrically insulating cylinder frame. In Examples of Patent Literature 2, an insulating tube made of polyethylene is fit to a cylinder of a forming die, and a positive material is put therein and formed by applying pressure; thereafter, a solid electrolyte powder is put therein and formed into a pellet by applying pressure. Then, a lithium foil, which will serve as the negative electrode, is press-bonded to the solid electrolyte side of the thus-obtained pellet.